In digital transmission systems, of both the land line and radiated wave varieties, the data payload is commonly transmitted in the form of a multilevel analog signal to improve the data transmission rate. In such systems each possible amplitude level represents a unique combination of data bits. Since there are 2.sup.n unique combinations of n bits, 2.sup.n amplitude levels are required to transmit n bits per baud. That bit rate can be doubled by quadrature amplitude modulating a high carrier frequency by each of two multilevel signals. A 16 QAM signal, therefore, uses four amplitude levels to transmit two data bits on each of two "rails" for a total of four data bits per baud.
Needless to say, the desire for higher and higher data transmission rates is driving the need for more amplitude levels; more levels, however, bring more complications. At the receiver or regenerator, a decoder must be able to accurately determine the transmitted levels in spite of any distortion or interference in order to recover the data. This can become very difficult, for example, in a 256 QAM system where sixteen discreet levels must be accurately discerned, to recover eight bits per baud.
In radio transmission systems, where continually changing atmospheric conditions cause varying amounts of signal fading, accurate quantizing or "slicing" of the multilevel signal is particularly difficult.
The traditional apparatus used to compensate for radio signal fading is the automatic gain control (AGC) circuit. A typical AGC circuit determines the amplitude of the envelope signal at radio frequency (RF) or intermediate frequency (IF) and compares it with a predetermined reference voltage to generate a difference signal, which drives an amplifier in a feedback circuit. Unfortunately, such AGC circuits are not nearly accurate enough for very high bit rate digital radio systems.
One known system for improving the quantizing accuracy in a digial radio receiver is disclosed in U.S. Pat. No. 4,326,169 entitled "Adaptive Decision Level Circuit", which issued to Fenderson et al. Apr. 20, 1982. According to the Fenderson scheme, the decision levels are shifted to compensate for shifts in signal levels. An error signal is generated by comparing the integrated difference between the binary output of a level slicer and its complement with the statistically expected difference. Feedback circuitry shifts the decision level to minimize the difference between the measured and expected values.
While the Fenderson scheme is very useful for radio systems in which up to eight amplitude levels are transmitted, e.g., 64 QAM, it does not provide enough improvement for reliable operation at sixteen amplitude levels.
An object of my invention is a much more accurate multilevel signal amplitude. used with fixed decision levels, it can enable accurate decoding of 16 amplitude levels in a 256 QAM digital radio system.